Oxide semiconductor, thin film transistor array substrate and production method thereof, and display device

ABSTRACT

The present invention provides an oxide semiconductor capable of achieving a thin film transistor having stable transistor characteristics, a thin film transistor having a channel layer formed of the oxide semiconductor and a production method thereof, and a display device equipped with the thin film transistor. The oxide semiconductor of the present invention is an oxide semiconductor for a thin film transistor. The oxide semiconductor includes indium, gallium, zinc, and oxygen as constituent atoms, and the oxygen content of the oxide semiconductor is 87% to 95% of the stoichiometric condition set as 100%, in terms of atomic units.

TECHNICAL FIELD

The present invention relates to an oxide semiconductor, a thin filmtransistor array substrate and a production method thereof, and adisplay device. Specifically, the present invention relates to an oxidesemiconductor suitable as a channel layer of a thin film transistor, athin film transistor array substrate provided with a thin filmtransistor having a channel layer formed by including the oxidesemiconductor, a production method of the thin film transistor arraysubstrate, and a display device equipped with the thin film transistorarray substrate.

BACKGROUND ART

Oxide semiconductors are semiconductor materials having higher electronmobility than silicon-based materials such as amorphous silicon (a-Si).For example, it is considered that use of oxide semiconductors forchannel layers of thin film transistors (TFT) can produce highlycredible TFTs with low leakage current when no voltage is applied. TFTsincluding oxide semiconductors have thus been developed.

Quaternary oxide semiconductors (hereinafter also referred to as IGZOsemiconductor) containing indium (In), gallium (Ga), zinc (Zn), andoxygen (O) as constituent atoms are considered more suitable for TFTthan other oxide semiconductors because they have not only high mobilitybut also such characteristics as mentioned below.

First, IGZO semiconductors can be formed into a film at relatively lowtemperatures around room temperatures to about 150° C. In the case ofusing the aforementioned silicon materials, a film of TFT is formed athigh temperatures of not lower than 300° C. Therefore, a TFT cannot bedirectly formed on a base material which is inappropriate for a hightemperature atmosphere, such as flexible substrates including a filmbase material. However, use of an oxide semiconductor makes it possibleto form TFT directly on a flexible substrate. Moreover, an IGZOsemiconductor can be formed into a film by a sputtering system and thuscan be produced by simple procedures.

Furthermore, thin films formed of an IGZO semiconductor can transmitvisible light and are thus excellent in transparency. For this reason,such films can be applied for use in transparent electrodes includingindium tin oxide (ITO) or the like.

Meanwhile, characteristics of IGZO semiconductors change depending onthe composition of constituent atoms. Patent Document 1 discloses aphase diagram showing preferable compositions of constituent atoms ofoxide semiconductors which are suitable as channel layers of TFTs.Patent Document 2 discloses preferable compositions of constituent atomsof oxide semiconductors which are suitable as transparent electrodes,such as TFT.

PRIOR ART REFERENCES Patent Documents

-   Patent Document 1: Japanese Patent Application Publication No.    2007-281409-   Patent Document 2: Japanese Patent Application Publication No.    2000-44236

DISCLOSURE OF THE INVENTION

With regard to IGZO semiconductors (oxide semiconductors) having thecompositions described in Patent Documents 1 and 2, the semiconductorlayer formed by including the IGZO semiconductor itself is excellent inmobility, transparency, or the like. However, if the IGZO semiconductoris applied as a channel layer of a TFT, stable and superior transistorcharacteristics cannot be maintained in some cases.

This is considered to be due to the following reasons: A TFT includesthree terminals of a signal electrode, a drain electrode, and a gateelectrode. On/Off operation of the TFT is performed by passing anelectric current through a region called a channel layer which isprovided between the signal electrode and the drain electrode, whilecontrolling the current by the voltage applied to the gate electrode. AnIGZO semiconductor constitutes a channel layer, and the signal electrodeand the drain electrode are formed after formation of the channel layer.Further, a protective layer for protecting the TFT or an interlayerinsulating film for flattening the surface of the substrate having TFTformed thereon are formed.

IGZO semiconductors can be formed into a film at relatively lowtemperatures as mentioned earlier. However, a higher temperature thanthe IGZO semiconductor film formation temperature is necessary forformation of electrodes, protective layers, interlayer insulating films,or the like, and a high temperature of 200° C. or higher is sometimesnecessary especially for formation of protective layers and interlayerinsulating films.

An IGZO semiconductor contains In, Ga, Zn, and O as constituent atoms.If the IGZO semiconductor is subjected to a temperature higher than thefilm formation temperature, desorption of oxygen contained in the filmoccurs so that the oxygen content changes. When the oxygen desorptionoccurs, the composition of the film is largely different from the IGZOsemiconductor composition (stoichiometry). As a result, some phenomenaoccur, such as increase of the off-current, reduction of the electronmobility, and hysteresis in the transistor properties, partly leading tofailure to achieve stable TFT characteristics.

The foregoing description exemplified the case of using an IGZOsemiconductor as a channel layer of a TFT. If an IGZO semiconductor isapplied in other fields, the oxygen content of the actually producedfilm that is formed of an IGZO semiconductor is also sometimes largelydifferent from the stoichiometric oxygen content.

Patent Document 1 describes the oxygen content of an oxidesemiconductor; however, the composition is determined by fluorescentX-ray analysis. This analysis can analyze surfaces of the film (sample)but does not have resolution in the thickness direction. Therefore, theanalysis can specify the composition of In, Ga, and Zn but cannotprecisely determine the amount of oxygen contained in the entire film.For this reason, the oxygen content described in Patent Document 1 isconsidered the amount of oxygen calculated based on the (stoichiometric)composition of the film (semiconductor layer) formed by using an IGZOsemiconductor, not the amount of oxygen obtained based on thecomposition of the constituent atoms of the produced film.

Patent Document 2 refers to the amount of oxygen loss. The amount ofoxygen loss relates to cations and thus is not uniquely determined.Further, oxygen loss is defined by the amount of carrier electrons.Therefore, unlike the oxygen content described herein, the amount ofoxygen loss cannot be quantitatively determined.

The present invention has been devised in consideration of theaforementioned current situation, and aims to provide an oxidesemiconductor capable of achieving a thin film transistor having stabletransistor characteristics, a thin film transistor including a channellayer formed of the oxide semiconductor and a production method thereof,and a display device equipped with the thin film transistor.

The present inventors made various investigations on an oxidesemiconductor capable of achieving a thin film transistor having staletransistor characteristics, and firstly focused their attention on thefact that oxide semiconductors containing In, Ga, and Zn are materialshaving high mobility and capable of providing transistor characteristicswith high credibility (resistance to stress). They also focused theirattention on the fact that the composition of an IGZO semiconductor filmafter made into a product is different from the stoichiometriccomposition, and that the difference is derived from oxygen desorptionfrom the IGZO semiconductor film caused by heating in the productionprocess. Then, the present inventors found that an IGZO semiconductorsuitable as a channel layer of a thin film transistor can be obtained bycontrolling the oxygen content in the IGZO semiconductor, and that thethin film transistor including the IGZO semiconductor has highlycredible transistor characteristics. Thereby, they have found that theforegoing problems can be solved, and accordingly achieved the presentinvention.

Namely, the present invention relates to an oxide semiconductor for athin film transistor, and the oxide semiconductor includes indium,gallium, zinc, and oxygen as constituent atoms, and the oxygen contentof the oxide semiconductor is 87% to 95% of the stoichiometric conditionset as 100%, in terms of atomic units.

The oxide semiconductor containing indium, gallium, zinc, and oxygen asconstituent atoms has high mobility and can be made into a film atrelatively low temperatures. Further, a film formed by including theoxide semiconductor has an excellent transparency.

The oxide semiconductor of the present invention can be suitably used asa channel layer of TFTs, transparent electrodes, or the like, bycontrolling the oxygen content of the oxide semiconductor to 87% to 95%of the stoichiometric condition set as 100%, in terms of atomic units.Especially in the case where the oxide semiconductor is used as achannel layer of TFTs, stable transistor characteristics can beachieved. The oxygen content of less than 87% tends to reduce thevoltage-current properties of the TFTs. The oxygen content of more than95% results in excessive resistance of the IGZO film, and thus the oxidesemiconductor tends not to function as a channel of the TFTs.

Meanwhile, the oxygen content of the oxide semiconductor can bedetermined by composition analysis such as auger electron spectroscopy(AES) and X-ray photoelectron spectroscopy (XPS).

As used herein, “stoichiometric condition” refers to a condition inwhich the charge number of metal ions is equal to the charge number ofoxygen ions. Therefore, the oxide semiconductor in the stoichiometriccondition does not have conductivity. Moreover, stoichiometry refers tothe film composition in an ideal state.

The present invention also relates to a thin film transistor arraysubstrate, including a substrate, and a thin film transistor mounted ona main surface of the substrate, and the thin film transistor includes achannel layer formed of the oxide semiconductor of the presentinvention. As mentioned earlier, a highly credible thin film transistorcan be provided if the thin film transistor includes a channel layerformed of the oxide semiconductor of the present invention having thecontrolled oxygen content.

The electron mobility of the thin film transistor is not particularlylimited but is preferably not less than 0.1 cm²/Vs. The electronmobility of this level can provide favorable transistor characteristics.

In the thin film transistor array substrate of the present invention,preferably, the thin film transistor further includes a protective layercovering the channel layer, and the protective layer contains oxygenatom-containing materials. The oxygen content of the channel layer canbe controlled in the aforementioned range by the oxygen atom containedin the protective layer.

The present invention also relates to a display panel equipped with thethin film transistor array substrate of the present invention. Examplesof the applicable display device include various display devices havinga thin film transistor array substrate, such as liquid crystal displaydevices, organic EL display devices, plasma display devices, and fieldemission display devices.

As mentioned earlier, the thin film transistor array substrate of thepresent invention has stable transistor characteristics. Therefore,display devices provided with the thin film transistor array substratehave high display quality.

The present invention further relates to a method of producing a thinfilm transistor array substrate. Namely, the present invention relatesto a method of producing a thin film transistor array substrateincluding a substrate and a thin film transistor mounted on a mainsurface of the substrate. The method includes the steps of forming aninsulating film covering a scanning wiring formed on a main surface ofthe substrate, forming a semiconductor layer to form an oxidesemiconductor layer at a position overlapping the scanning wiring uponseeing a substrate surface from a normal direction, forming a wiring andan electrode to form a signal wiring and a drain electrode on the oxidesemiconductor layer, forming a protective layer covering the signalwiring and the drain electrode, and forming an interlayer insulatingfilm covering the protective layer. A baking treatment is furtherincluded between the step of forming a protective layer and the step offorming an interlayer insulating film.

In the step of forming the insulating film, the scanning wiring partlyfunctions as a gate electrode of the thin film transistor. The scanningwiring is preferably formed of low-resistance metal materials such astitanium (Ti), aluminum (Al), and copper (Cu), and may be a laminatefilm of such metal materials. For example, the scanning wiring is formedby depositing the metal materials by sputtering on the main surface ofthe glass substrate to form a laminate film, followed by patterning by aphotolithographic method including wet-etching step and resist-peelingstep.

The insulating layer is formed by, for example, forming inorganicmaterials such as silicon oxide (SiOx) and silicon nitride (SiNx) into afilm by chemical vapor deposition (CVD) method or the like.

In the step of forming a semiconductor layer, a semiconductor layer isformed by including the oxide semiconductor of the present invention.The method of forming the semiconductor layer is not particularlylimited. Examples of the method include a method of firstly forming afilm of the oxide semiconductor by sputtering, and then patterning theformed film in a desired shape by photolithographic method. If such amethod is employed, various agents such as etching liquids and resistremoving liquids are used in the patterning step.

In the step of forming a wiring and an electrode, a signal wiring and adrain electrode are formed on the oxide semiconductor layer. Thestructures of the signal wiring and the drain electrode are the same asthose of the scanning wiring.

In the step of forming a protective layer, a protective layer coveringthe signal wiring and the drain electrode is formed. The protectivelayer is formed by, for example, forming inorganic materials such assilicon oxide (SiOx) and silicon nitride (SiNx) into a film by CVDmethod, or the like.

In the step of forming an interlayer insulating film, an interlayerinsulating film covering the protective layer is formed. The interlayerinsulating film is formed by, for example, including photosensitiveresins.

Meanwhile, according to the present invention, a baking treatment isperformed between the step of forming a protective layer and the step offorming an interlayer insulating film in order to control the oxygencontent of the semiconductor layer. The baking treatment after formingthe protective layer makes it possible to supply oxygen to the oxidesemiconductor layer through the protective layer. Alternatively, theoxygen content of the oxide semiconductor layer can be controlled bysupplying oxygen to the oxide semiconductor layer from the insulatinglayer provided on the lower side of the oxide semiconductor layer andfrom the protective layer provided on the upper side of the oxidesemiconductor layer. The oxygen content is not particularly limited butis preferably not less than 90% because such an amount reduces thedifference from the stoichiometric condition, and thus stable transistorcharacteristics can be achieved.

In the method of producing the thin film transistor array substrate ofthe present invention, the baking treatment is preferably performed atthe highest treatment temperature (220° C. or higher) among thetreatment temperatures in the production steps of the thin filmtransistor array substrate. This arrangement makes it possible to easilyand assuredly control the oxygen content of the semiconductor layer.

Each of the aforementioned embodiments may be appropriately combined ina scope not departing from the principles of the present invention.

EFFECTS OF THE INVENTION

The oxide semiconductor of the present invention has a predeterminedoxide content. Therefore, if the oxide semiconductor is used for exampleas a channel layer of a thin film transistor, a highly credible thinfilm transistor with stable transistor characteristics can be achieved.Moreover, a display device including a thin film transistor arraysubstrate equipped with the thin film transistor can display highquality images. Furthermore, the method of producing a thin filmtransistor array substrate of the present invention includes a bakingtreatment performed after formation of a semiconductor layer. As aresult, the oxygen content in the semiconductor layer can be easilycontrolled.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic vertical cross-sectional diagram showing astructure of a pixel of a liquid crystal display device according to afirst embodiment.

FIG. 2 is a schematic plain diagram showing a structure of a TFT arraysubstrate in the liquid crystal display device according to the firstembodiment.

FIG. 3 is a schematic cross sectional diagram showing a step ofproducing the TFT array substrate according to the first embodiment.

FIG. 4 is a schematic cross sectional diagram showing a step ofproducing the TFT array substrate according to the first embodiment.

FIG. 5 is a schematic cross sectional diagram showing a step ofproducing the TFT array substrate according to the first embodiment.

FIG. 6 is a schematic cross sectional diagram showing a step ofproducing the TFT array substrate according to the first embodiment.

FIG. 7 is a schematic cross sectional diagram showing a step ofproducing the TFT array substrate according to the first embodiment.

FIG. 8 is a schematic plain diagram showing a step of producing a CFsubstrate included the liquid crystal display device according to thefirst embodiment.

FIG. 9 is a schematic plain diagram showing a step of producing a CFsubstrate included the liquid crystal display device according to thefirst embodiment.

FIG. 10 is a schematic plain diagram showing a step of producing a CFsubstrate included the liquid crystal display device according to thefirst embodiment.

FIG. 11 is a graph showing the composition and the oxygen contents ofthe oxide semiconductor included in channel layers of the TFTs accordingto Examples 1 and 2, and Comparative Example 1.

FIG. 12 is a graph showing electric properties of a TFT of a displaydevice according to Example 1.

FIG. 13 is a graph showing electric properties of a TFT of a displaydevice according to Example 2.

FIG. 14 is a graph showing electric properties of a TFT of a displaydevice according to Comparative Example 1.

MODES FOR CARRYING OUT THE INVENTION

The present invention will be described in detail below by showingembodiments and referring to drawings. The present invention is notlimited only to those embodiments.

Embodiment 1

FIG. 1 is a schematic vertical cross-sectional diagram showing astructure of a pixel of a liquid crystal display device according to thepresent embodiment. In FIG. 1, a liquid crystal display device 10 isprovided with a TFT array substrate 11 having a TFT formed therein, acolor filter (CF) substrate 13 as a counter substrate disposed facingthe TFT array substrate 11, and a liquid crystal layer 12 sandwichedbetween the above two substrates.

FIG. 2 is a schematic plain diagram showing the structure of the TFTarray substrate 11 in the liquid crystal display device of the presentembodiment. In FIG. 2, scanning wirings 102 and signal wirings 106 aredisposed in a grid pattern on the main surface of a glass substrate 101.In a plurality of pixel areas which are sectioned by the scanningwirings 102 and the signal wirings 106, TFTs 15 as a switch element areformed in the vicinities of the intersections of the scanning wirings102 and the signal wirings 106.

In a detailed look of the areas where the TFTs 15 are formed, as shownin FIG. 1, the main surface of the glass substrate 101 having thescanning wirings 102 formed thereon is covered by a gate insulating film103 as an insulating layer. An IGZO semiconductor layer 104 is formed onthe gate insulating film 103 in a manner overlapping the scanning wiring102. The signal wiring 106 and a drain electrode 107 are formed on theIGZO semiconductor layer 104. The TFT 15 is covered with an interlayerinsulating film 109 to flatten the protective layer 108 and thesubstrate surface. A pixel electrode 110 is formed on the interlayerinsulating film 109.

The TFT 15 includes the scanning wiring 102 partly as a gate electrode,the gate insulating film 103, the IGZO semiconductor layer 104 as achannel layer, the signal wiring 106, and the drain electrode 107.

The scanning wiring 102 has a structure including laminated scanningwiring layers 102 a, 102 b, and 102 c. An applicable example of thescanning wiring 102 includes a scanning wiring which has a laminatestructure consisting of the scanning wiring layers 102 a and 102 cformed of Ti and having a thickness of 30 to 150 nm, and the scanningwiring layer 102 b formed of Al and having a thickness of 200 to 500 nm.

Inorganic materials such as silicon oxide (SiOx) and silicon nitride(SiNx) are applicable for the gate insulating film 103. The thickness ofthe gate insulating film 103 is set for example to around 200 to 500 nm.

The IGZO semiconductor layer 104 is formed of an IGZO semiconductorwhich includes In, Ga, Zn, and O as constituent atoms, and has theoxygen content of 87% to 95% of the stoichiometric condition set as100%, in terms of atomic units. If the channel layer of the TFT 15 isformed of the IGZO semiconductor having the aforementioned oxygencontent, the TFT array substrate 11 having stable transistorcharacteristics can be obtained. The thickness of the IGZO semiconductorlayer 104 is not particularly limited, and is around 10 to 300 nm.

The signal wiring 106 partly functions as a source electrode of the TFT15. As an example of the signal wiring 106, a signal wiring having astructure consisting of laminated signal wiring layers 106 a and 106 bis exemplified. The drain electrode 107 has a structure consisting oflaminated drain electrode layers 107 a and 107 b. Material of the signalwiring 106 and that of the drain electrode 107 may be the same ordifferent from one another.

Examples of the signal wiring 106 and the drain electrode 107 include asignal wiring and a drain electrode each having a laminated structure inwhich the signal wiring layer 106 a and the drain electrode layer 107 aare formed of Ti, and the signal wiring layer 106 b and the drainelectrode layer 107 b are formed of Al. The thickness of the signalwiring layer 106 a and the drain electrode layer 107 a is for examplearound 30 to 150 nm, and the thickness of the signal wiring layer 106 aand the drain electrode layer 107 b is for example around 50 to 400 nm.

As the protective layer, a layer prepared by forming inorganic materialssuch as SiOx and SiNx into a film by CVD method, sputtering method, orthe like can be applicable. The protective layer may include not only asingle film of the SiOx film or the SiNx film, but a laminate of theSiOx film and the SiNx film as well. The interlayer insulating film isformed by, for example, including photosensitive resins.

The pixel electrode 110 is formed of a transparent electrode materialsuch as ITO, and the thickness thereof is around 50 to 200 nm.

Meanwhile, the CF substrate 13 has a red (R) CF layer 203, a blue (B) CFlayer 203, or a green (G) CF layer 203 in each pixel area on the mainsurface of a glass substrate 201 as shown in FIG. 1. The CF layers 203of respective colors are sectioned by a light-shielding member (notshown) called black matrix. A counter electrode 204 having a thicknessof around 50 to 200 nm is formed on the surface of the substrate. Thearea having the light-shielding member formed thereon is provided with aphotospacer (not shown).

An example of a method of producing the liquid crystal display device 10having the aforementioned structure is described with some concreteexamples below. First, an example of a method of producing the TFT arraysubstrate 11 is explained with reference to FIG. 3 to FIG. 7. FIG. 3 toFIG. 7 are schematic cross sectional diagrams each showing a step ofproducing the TFT array substrate 11 of the present embodiment.

FIG. 3 shows a state in which the scanning wiring 102 is formed on themain surface of the glass substrate 101. For providing the scanningwiring 102, a Ti film having a thickness of 30 to 150 nm, an Al filmhaving a thickness of 200 to 500 nm, and Ti film having a thickness of30 to 150 nm are formed in said order on the main surface of the glasssubstrate 101 by, for example, a sputtering method so that the scanningwiring layers 102 a, 102 b, and 102 c are formed. Next, the resultinglaminated film is patterned into a desired shape by photolithographicmethods (hereinafter, referred simply to as photolithography method)including wet-etching treatment and resist-peeling treatment. Thereby,the scanning wiring 102 can be provided.

FIG. 4 shows a state in which the main surface of the substrate in thestate shown in FIG. 3 is covered with the insulating film 103, andfurther the IGZO semiconductor layer 104 is formed thereon. Thesubstrate in this state can be obtained by performing the insulatingfilm forming step and subsequently the semiconductor layer forming step.

In the insulating film forming step, the gate insulating film 103 isformed by depositing SiO₂ by a CVD method in a manner to have athickness of 200 to 500 nm and to cover the glass substrate 101 and thescanning wiring 102. In the semiconductor layer forming step, the IGZOsemiconductor layer 104 is formed at the position overlapping thescanning wiring 102 when seeing the substrate surface from a normaldirection.

The IGZO semiconductor layer 104 can be obtained by firstly depositing atarget including In—Ga—Zn—O by sputtering under at an output of 0.1 to2.0 kW such that a resulting layer has a thickness of 10 to 300 nm, andthen pattern-forming the layer into a desired shape by aphotolithographic method. The composition ratio of the target may be,for example, In:Ga:Zn:O=1:1:1:4, or In:Ga:Zn:O=2:2:1:7. However, thepresent invention is not limited to those examples, and the compositionratio of the target may be appropriately set dependent on thefilm-forming condition.

FIG. 5 shows the state of the substrate after the wiring forming stepand the electrode forming step. In the wiring forming step and theelectrode forming step, the signal wiring 106 and the drain electrode107 are further formed on the substrate in the state shown in FIG. 4.First, Ti is deposited to have a thickness of 30 to 150 nm by asputtering method so that the signal wiring layer 106 a and the drainelectrode layer 107 a are formed. Next, Al is deposited to have athickness of 50 to 400 nm so that the signal wiring layer 106 b and thedrain electrode layer 107 b are formed. The thus obtained laminatedlayers of Ti and Al are subjected to patterning by a photolithographicmethod, and thereby the signal wiring 106 and the drain wiring 107 areformed.

FIG. 6 shows the state of the substrate after the protective layerforming step and the interlayer insulating film forming step. In theprotective layer forming step, SiO₂ is deposited to have a thickness of100 to 700 nm by a CVD method so that the protective layer 108 coveringboth of the signal wiring 106 and the drain electrode 107 is formed.

In the interlayer insulating film forming step, the interlayerinsulating film 109 containing a photosensitive resin is formed in amanner to cover the protective layer 108.

FIG. 7 shows the state after the pixel electrode 110 is formed. Thesubstrate in such a condition can be obtained by, for example, firstlydepositing ITO to form a thin film having a thickness of 50 to 200 nm bysputtering on the interlayer insulating film 109, and then subjectingthe thin film to sputtering into a desired shape by photolithographicmethod so that the pixel electrode 110 is formed.

In this embodiment, a baking treatment step for baking the protectivelayer 108 is further included between the protective layer forming stepand the interlayer insulating film forming step shown in FIG. 6. In theliquid crystal display device 10 of the present invention, the IGZOsemiconductor layer 104 is covered by the protective layer 108, and isattached to the CF substrate 13 through the interlayer insulating filmforming step as described later so that the IGZO semiconductor layer 104is completely shut off from outside. In such a state, oxygen desorptionof the IGZO semiconductor layer 104 is considered to occur between theIGZO semiconductor layer 104 and the gate insulating film 103, and/orbetween the IGZO semiconductor layer 104 and the protective layer 108.

Considering the above, in the present embodiment, the baking treatmentis performed at the highest temperature among the treatment temperaturesin the respective production steps after forming the protective layer108. In the respective production steps, the IGZO semiconductor layer104 is formed at about room temperatures to about 150° C., whereas thegate insulating film 103, the protective layer 108, and the interlayerinsulating film. 109 to be described later are formed at around 200° C.to 220° C. Therefore, the baking treatment is performed at a temperaturehigher than the temperature in forming the gate insulating film. 103,the protective layer 108, and the interlayer insulating film 109 (220°C. or higher), and thereby the oxygen content of the IGZO semiconductorlayer 104 is controlled. The method of the baking treatment is notparticularly limited, and a simple treatment of baking with a clean ovenunder air atmosphere may be employed.

Accordingly, oxygen is supplied to the IGZO semiconductor layer 104 viathe protective layer 108. Alternatively, oxygen is supplied to the IGZOsemiconductor layer 104 via the gate insulating film. 103 and theprotective layer 108. As a result, the oxygen concentration of the IGOsemiconductor layer 104 becomes stable, and also oxygen desorption ofthe IGZO semiconductor layer 104 is avoided. Thereby the IGZOsemiconductor layer 104 having a desired oxygen content can be achieved.The oxygen content may vary depending on the desired transistorcharacteristics to be obtained. The oxygen content of 90% or morereduces the difference from the oxygen content in the stoichiometriccondition, and thus stable transistor characteristic can be achieved.

One example of the method of producing the CF substrate 13 is explainedwith reference to FIG. 8 to FIG. 10. FIG. 8 to FIG. 10 are eachschematic plain diagram showing a step of producing a CF substrateincluded in the liquid crystal display device according to the firstembodiment. First, as shown in FIG. 8, a photosensitive resin includinga black pigment is patterned in a desired shape on the main surface ofthe glass substrate 201 by a photolithographic method to form a lightshielding member 202. Next, photosensitive resins each containing a redpigment (R), a green pigment (G), or a blue pigment (B) are applied inthe regions sectioned by the light shielding members 202 so that CFlayers 203R, 203G, and 203B are formed.

As shown in FIG. 9, a transparent electrode material such as ITO isdeposited to have a thickness of 50 to 200 nm on the surface of thesubstrate by sputtering. Thereafter, by a photolithographic method, thecounter electrode 204 having a desired pattern is formed. Further, asshown in FIG. 10, photospacers 205 are formed at the regions where thelight shielding members 202 are formed. The photospacers 205 can beobtained by using photosensitive resins and patterning in a desiredshape by a photolithographic method.

A polyimide resin is applied on the surfaces of the above produced TFTarray substrate 11 and the CF substrate 13 by a printing method to formalignment films (not shown). The thus obtained both substrates areattached each other with a sealing material in between. Then, liquidcrystals are filled in between the substrates by a dropping method, aninjection method, or the like. The attached substrates were subjected todicing to be divided, followed by mounting of necessary items such as adriving device, a casing, and a light source, and thereby a liquidcrystal display device 10 of the present embodiment is obtained.

In the present embodiment, an IGZO semiconductor having a specific atomcomposition is formed as mentioned earlier. Moreover, after theprotective layer 108 is formed, a baking treatment is performed at thehighest temperature among the temperatures in the above protectionsteps. As a result, the oxygen concentration of the IGZO semiconductorlayer 104 becomes stable, and oxygen desorption of the IGZOsemiconductor layer 104 does not occur after the baking treatment.Thereby a highly credible liquid crystal display device 10 can beprovided.

Specific examples of the liquid crystal display device 10 of the presentembodiment is described below.

Example 1

In the liquid crystal display device 10 according to the firstembodiment, an IGZO semiconductor layer having a thickness of 50 nm wasformed by using a sputtering target having the In:Ga:Zn:O ratio=1:1:1:4.

In order to control the oxygen content of the IGZO semiconductor layer104, a baking treatment was performed at 350° C. in the atmosphere forone hour after forming the protective layer 108. A clean oven was usedherein for the one-hour baking treatment at 350° C. in the atmosphere.

The composition of the constituent atoms of the IGZO semiconductor layer104 at a depth of about 20 nm from the surface thereof was measured byAES (Auger Electron Spectroscopy) analysis. The AES analysis wasconducted with an AES analyzer (produced by JEOL Ltd., Model No.JAMP-9500F) in the following measurement conditions. Electronirradiation condition: 5 kV, 5 nA; Sample: 75 degrees inclination;Neutralization condition: Ar ion 10 eV, 1 μA; Energy resolution ofdetector: dE/E=0.35%, Detection energy step: 1.0 eV. Detected peaks forrespective constituent atoms of In, Ga, Zn, O, and Si were obtained.

The AES analysis was explained here in detail. The AES analysis isperformed by irradiating a measuring target spot of a sample withelectron beams, and obtaining the spectrum based on the kinetic energyand the detected intensity of the auger electron emitted from thesurface. Since a peak location and a shape of a spectrum are unique toeach element, the element is identified based on the peak location andthe shape of the spectrum. The concentration of the element in thematerial is calculated from the intensity (amplitude) of the spectrum.Accordingly, the element analysis is performed. Further, since the peaklocation and the shape of the spectrum are unique to bonding state ofthe atom, chemical bonding states (oxidation state, or the like) of theelements can also be analyzed.

The Auger electron consists of a very small portion of a huge amount ofthe detected electron, and therefore notably receives backgroundinfluences of low frequency components. In this example, in order toobtain a more accurate oxygen content, Rutherford BackscatteringSpectrometry (RBS) and Particle Induced X-ray Emission (PIXE) were alsoperformed in addition to the AES analysis. The values obtained were usedto correct the sensitivity factor.

Namely, as is generally performed, the spectrum was differentiated toremove the backgrounds of the low frequency components. Then, thecomposition ratio was calculated from the peak intensities of therespective elements using the sensitivity factor (the values of pureelements accompanied with the device) unique to each element. Based onthe measurement result, the oxygen content was calculated by the generalformula mentioned later.

Meanwhile, the peak intensity and the shape of each element change ifthe chemical bonding state largely changes. For this reason, thesensitivity factor needs to be corrected to obtain the composition ratiowith higher accuracy. Therefore, the sensitivity factor is corrected asmentioned below upon calculating the composition ratio.

Specifically, in order to check the oxidation state or reduction stateof In, the abundance of In in the states of In (pure metal) andIn(In₂O₃) was calculated by a procedure mentioned later. Namely, thedifferentiated In spectrum obtained in the AES analysis was subjected tononnegative-constrained least squares fitting in the standardmeasurement peak of In (pure metal) and In (In₂O₃) to separate eachelement. The composition ratio was calculated using the sensitivityfactor (the values of pure elements accompanied with the device).

The oxygen content was obtained by the general formula below.O(atomic %)/{In(atomic %)×3/2+Ga(atomic %)×3/2+Zn(atomic %)}

By this calculation, the composition of the IGZO semiconductor includedin the channel layer of the TFT 15 was obtained as shown in a graph inFIG. 11. FIG. 11 is the graph showing the composition and the oxygencontent of the oxide semiconductor included in the channel layer of theTFT in Example 1, and Example 2 and Comparative Example 1 describedlater.

Further, TFT characteristics of the obtained TFT array substrate 11 weremeasured. For measuring the TFT characteristics, the threshold value(Vth), mobility (μ), and subthreshold swing value (S) were calculatedbased on the measurement result shown in the graph in FIG. 12. Thecharacteristics were judged based on the criteria for judging mentionedbelow. Each of the judging criteria needs to be satisfied by all theVth, μ, and S. Even one unsatisfied item leads to a judgment ofinappropriate.

++: 0 V<Vth<10 V, μ>5, S<1.5

+: −5 V<Vth<10 V, μ≧1, S≦2.5

−: −10 V<Vth<15 V, μ<1, S>2.5, or unmeasurable (−)

FIG. 12 is a graph showing the electric properties of the TFT of thedisplay device in Example 1. In the graph in FIG. 12, the bold lineshows the Vg−Id characteristics, and the thin line shows the Vg−√Idcharacteristics. Here, Vg refers to a gate voltage, and Id refers to adrain current.

The graph in FIG. 12 and Table 1 below show the obtained measurementresults.

TABLE 1 Vth[V] μ[cm²/Vs] S[V/dec] Judgement Example 1 6.89 7.67 1.27 ++Example 2 5.91 4.88 2.00 + Comparative — — — − Example 1

Example 2

The composition of the IGZO semiconductor was set to the values shown inthe graph in FIG. 11. The baking treatment was performed with a cleanoven at 220° C. for one hour in the same manner as in Example 1. Exceptfor the above, various physical characteristic values were measured inthe same manner as in Example 1.

Table 1 and FIG. 13 show the obtained measurement results. FIG. 13 is agraph showing the electric properties of the TFT of the display devicein Example 2.

Comparative Example 1

The composition of the IGZO semiconductor was set to the values shown inthe graph in FIG. 11. Baking treatment was not performed after formingthe protective layer 108. Except for the above, the physicalcharacteristic values were measured in the same manner as in Example 1.Table 1 and FIG. 14 show the obtained measurement results. FIG. 14 is agraph showing the electric properties of the TFT of the display devicein Comparative Example 1.

Table 1 and the graphs shown in FIGS. 12 to 14 reveal that the TFTs inExample 1 and Example 2 have excellent electric properties. It is alsoclarified that the TFT in Comparative Example 1 is inferior in the thinfilm transistor characteristics.

The above embodiment is explained with an example in which the lightshielding member 202 and the CF layer are provided on the countersubstrate side. However, the present invention is not limited to thismode, and those members may be formed on the TFT array substrate 11side.

The above embodiment is explained with an example in which the IGZOsemiconductor is used as the channel layer of the TFT. However, thepresent invention is not limited to this example, and the IGZOsemiconductor is applicable for transparent electrodes or the like.

Further, the above embodiments are explained by exemplifying the liquidcrystal display device. However, the present invention is not limited tothe examples, and is applicable for an organic EL display device, aplasma display device, a field emission display device, or the like.

The aforementioned embodiments in examples may be combined in a scopenot departing from the principles of the present invention.

The present application claims priority to Patent Application No.2009-154104 filed in Japan on Jun. 29, 2009 under the Paris Conventionand provisions of national law in a designated State, the entirecontents of which are hereby incorporated by reference.

EXPLANATION OF REFERENCE NUMERALS

-   -   10: Liquid crystal display device    -   11: TFT array substrate    -   12: Liquid crystal layer    -   13: CF substrate    -   15: TFT    -   101, 102: Glass substrate    -   102 a, 102 b, 102 c: Scanning wiring layer    -   102: Scanning wiring    -   103: Gate insulating film    -   104: Oxide semiconductor layer    -   106 a, 106 b: Signal wiring layer    -   106: Signal wiring    -   107 a, 107 b: Drain electrode layer    -   107: Drain electrode    -   108: Protective layer    -   109: Interlayer insulating film    -   110: Pixel electrode    -   121: Channel protective layer    -   202: Light shielding member    -   203R, 203G, 203B: CF    -   204: Counter electrode    -   205: Photospacer

The invention claimed is:
 1. A thin film transistor substratecomprising: a substrate body: a gate electrode provided on the substratebody; a gate insulating film provided on the gate electrode; an oxidesemiconductor film provided directly on the gate insulating film andincluding an overlapped portion that overlaps with the gate electrode; asource electrode provided on the oxide semiconductor layer; a drainelectrode provided on the oxide semiconductor film and opposed to thesource electrode at the overlapped portion; and a protective filmprovided on the overlapped portion of the oxide semiconductor film, thesource electrode, and the drain electrode, wherein the protective filmincludes material containing an oxygen atom and is in direct contactwith a portion of the oxide semiconductor film, the oxide semiconductorfilm is sandwiched by the gate insulating film and the protective filmin a thickness direction of the substrate body, the oxide semiconductorfilm includes indium, gallium, zinc and oxygen as constituent atoms, andthe oxygen content of the oxide semiconductor, which directly contactsboth the gate insulating film and the protective film, is 87% to 95% inaccordance with the following formula:O(atomic %)/{In(atomic %)×3/2+Ga(atomic %)×3/2+Zn(atomic %)}.
 2. Thethin film transistor according to claim 1, wherein a current that flowsthrough a channel layer of the oxide semiconductor film when no voltageis applied to the gate electrode is less than 10⁻⁸ A.
 3. The thin filmtransistor according to claim 2, wherein the protective film is CVDdeposited silicon oxide film.
 4. The thin film transistor according toclaim 2, wherein the current flow that flows through the channel layerwhen no voltage is applied to the gate electrode is less than 10⁻¹⁰ A.5. The thin film transistor according to claim 4, wherein the protectivefilm is CVD deposited silicon oxide film.